fix(adapter): Detect duplicate adapter names, reject indexers, validate internal constructor accessibility

[Copilot]

Addresses four issues in the adapter generator that could cause silent failures or confusing diagnostics:

Changes

• Duplicate adapter detection: Track generated adapter type names across multiple [GenerateAdapter] attributes to detect conflicts before AddSource() is called. Previously, duplicate names would cause AddSource() to throw without a helpful diagnostic.

• Indexer rejection: Added PKADP018 diagnostic when target contracts contain indexers. Previously indexers were silently filtered out, causing the generated adapter to fail compilation without indicating why.

• Internal constructor accessibility: Enhanced abstract class constructor validation to use IsSymbolAccessibleWithin() for internal constructors, correctly rejecting those from referenced assemblies.

• Documentation: Updated AnalyzerReleases.Unshipped.md and docs/generators/adapter.md with PKADP018.

Example

// Now reports PKADP006 with clear message
[GenerateAdapter(Target = typeof(IFoo), Adaptee = typeof(Bar))]
[GenerateAdapter(Target = typeof(IBaz), Adaptee = typeof(Qux), AdapterTypeName = "BarToIFooAdapter")]
public static partial class Adapters { } // Error: duplicate adapter name

// Now reports PKADP018 instead of silent failure
public interface IIndexable
{
    string this[int index] { get; } // Error: indexers not supported
}

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